Quantico Arms on the Industry link has RRA lowers for $85.00 each...
I think these are stripped lowers...
You also have overlooked DPMS - excellent lowers...
Also Stinger lowers (saw one at a gunshow - fine workmanship) - I think it sold for $89 stripped...
[sniper2]

"I know not what course others may take, but as for me, give me liberty, or give me death."

Well, if you put it that way, Colt IMO, as long as it is not one that has non-standard pins or a block. However you should consider what Cutter75 said. If it's not a Colt upper, then there is not much point to any of them but price. I say go with RRA and hell with the other choices. These ARE just KIT rifles after all. The only reason I could see with going with your choices is if you can get in on some sort of deal, though at $85 for a RRA, I don't see how you could be saving much.

"I'll never fully trust people with cheap guns. I think they're all moments away from being criminals." - Tweak

Scht0nk, are you talking about completed lowers or stripped lowers? If you are talking about completed lowers I'd spring for the Colt. If you are talking about stripped lowers then I'd skip the Colt and buy on price.
I love my Colt rifles but building one from the ground up is more expensive [b]unless[/b] you don't mind spending an extra $50 to buy .170 pin FCG parts. Full parts kits come with conventional size FCG parts, so you will have to buy a set that works with your Colt lower, (assuming that the Colt lower has large pins, and it probably does).
Believe it or not, the cheapest place to buy large pin Colt FCG parts is Bushmaster's online store. I bought three sets from Bushy and they are genuine Colt parts (at least mine were) [:)]